1. Field of the Invention
This invention relates generally to digital signal processing systems implementing adaptive feedback equalization, and particularly, to an initialization scheme for a hybrid type equalizer having a frequency domain equalizer in the forward path and a time-domain equalizer in the feedback path.
2. Discussion of the Prior Art
Decision Feedback Equalization is a technique used to eliminate all inter-symbol interference (ISI) caused by the transmission channel in digital communication systems. FIG. 1 is a schematic illustration of a typical Decision Feedback Equalizer (DFE) system 10. As shown in FIG. 1, the typical DFE includes a feed forward path including a first finite impulse response (FIR) filter 12, a feedback path 13 including a second FIR filter 14, a decision device 15, and, an error calculator 18. The input symbol xnrepresents the symbol inputs which are input to the first finite impulse response (FIR) filter 12. It is understood that first and second FIR filters 12, 14 are linear transversal filters each representing an adaptive transfer function f(n), g(n), respectively according to respective sets of adaptable coefficients fn, gn. In operation, the output of the fist FIR filter 12 is summed with the output of the feedback FIR filter 14 section to provide a desired DFE output represented as signal vn 20. In operation, the coefficients of each of the forward FIR filter 12 and feedback FIR filter 14 recursively adapt according an output error signal en 16 of the feedback path until some convergence factor or error metric, e.g., mean square error, is satisfied. As shown in FIG. 1, the output error signal en 16 of the feedback path represents the difference between an input reference signal 21, i.e. a desired output signal, and an intermediate output signal yn which is an output of decision block 15. As known to skilled artisans and described in “Adaptive Decision-Feedback Equalizer” in the book “Digital Communications” by John G. Proakis, McGraw-Hill, 1995, 3rd ed., Ch. 11-2. pages 650 et seq., (ISBN 0-07-05-51726-6), the whole contents and disclosure of which is incorporated by reference as is fully set forth herein, the equalizer coefficients are adjusted recursively in the adaptive mode of the DFE.
In such decision feedback equalizers, schemes are implemented that require a fairly good initial setup of equalizer taps (coefficients) to ensure that most of the (tentatively) decoded symbols are correct. Blind equalization algorithms permit estimation of the equalizer coefficients without any knowledge about the channel response or the data sent.
It would be highly desirable to provide an initialization scheme for a decision feedback equalizer of a special type herein referred to as a hybrid equalizer having a frequency domain equalizer in the forward path and a time-domain equalizer in the feedback path.